This invention relates to thermally developable imaging materials such as photothermographic materials. More particularly, it relates to photothermographic imaging materials that are sensitive to infrared imaging radiation and exhibit improved stability to heat over time. The invention also relates to methods of imaging using these materials. This invention is directed to the photothermographic imaging industry.
Silver-containing photothermographic imaging materials that are developed with heat and without liquid development have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, visible, ultraviolet or infrared radiation) and developed by the use of thermal energy. These materials, also known as xe2x80x9cdry silverxe2x80x9d materials, generally comprise a support having coated thereon: (a) photosensitive catalyst (such as silver halide) that upon such exposure provides a latent image in exposed grains that is capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In such materials, the photosensitive catalyst is generally a photographic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms, (Ag0)n, also known as silver specks, clusters, nuclei, or latent image, are generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms [Klosterboer, Imaging Processes and Materials (Neblette""s Eighth Edition), Sturge, Walworth and Shepp (Eds.), Van Nostrand-Reinhold, New York, Chapter 9, pp. 279-291, 1989]. It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed in catalytic proximity with the non-photosensitive source of reducible silver ions in a number of different ways (see, for example, Research Disclosure, June 1978, item 17029). Other photosensitive materials, such as titanium dioxide, cadmium sulfide, and zinc oxide, have also been reported to be useful in place of silver halide as the photocatalyst in photothermographic materials [see for example, Shepard, J. Appl. Photog. Eng. 1982, 8(5), 210-212, Shigeo et al., Nippon Kagaku Kaishi, 1994, 11, 992-997, and FR 2,254,047 (Robillard)].
The photosensitive silver halide may be made xe2x80x9cin situxe2x80x9d, for example, by mixing an organic or inorganic halide-containing source with a source of reducible silver ions to achieve partial metathesis and thus causing the in situ formation of silver halide (AgX) grains throughout the silver source [see, for example, U.S. Pat. No. 3,457,075 (Morgan et al.)]. In addition, photosensitive silver halides and sources of reducible silver ions can be coprecipitated [see Usanov et al., J. Imag. Sci. Tech. 40, 104 (1996)]. Alternatively, a portion of the reducible silver ions can be completely converted to silver halide, and that portion can be added back to the source of reducible silver ions (see Usanov et al., International Conference on Imaging Science, Sep. 7-11, 1998)
The silver halide may also be xe2x80x9cpreformedxe2x80x9d and prepared by an xe2x80x9cex situxe2x80x9d process whereby the silver halide (AgX) grains are prepared and grown separately. With this technique, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to both the silver halide grains and the photothermographic material. The preformed silver halide grains may be introduced prior to, and be present during, the formation of the source of reducible silver ions. Co-precipitation of the silver halide and the source of reducible silver ions provides a more intimate mixture of the two materials [see for example, U.S. Pat. No. 3,839,049 (Simons)]. Alternatively, the preformed silver halide grains may be added to and physically mixed with the source of reducible silver ions.
The non-photosensitive source of reducible silver ions is a material that contains reducible silver ions. Typically, the preferred non-photosensitive source of reducible silver ions is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, or mixtures of such salts. Such acids are also known as xe2x80x9cfatty acidsxe2x80x9d or xe2x80x9cfatty carboxylic acidsxe2x80x9d. Silver salts of other organic acids or other organic compounds, such as silver imidazoles, silver tetrazoles, silver benzotriazoles, silver benzotetrazoles, silver benzothiazoles and silver acetylides have also been proposed. U.S. Pat. No. 4,260,677 (Winslow et al.) discloses the use of complexes of various inorganic or organic silver salts.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a xe2x80x9cdeveloperxe2x80x9d, may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent for silver ion. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer(s).
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
As noted above, in photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50xc2x0 C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30xc2x0 C. to 50xc2x0 C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the xe2x80x9cchemistryxe2x80x9d for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called xe2x80x9cinstant photographyxe2x80x9d, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of xe2x80x9cfogxe2x80x9d or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems during the preparation of the photothermographic emulsion as well as during coating, use, storage, and post-processing handling.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is, in the aqueous fixing step).
In photothermographic materials, the binder is capable of wide variation and a number of binders (both hydrophilic and hydrophobic) are useful. In contrast, conventional photographic materials are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette""s Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94-103, and in M. R. V. Sahyun, J. Imaging Sci. Technol., 1998, 42, 23.
Thermally developable materials have gained widespread use in several industries, particularly in radiography.
A common problem that may exist with some photothermographic imaging materials is xe2x80x9cpost-processingxe2x80x9d instability of the image (that is, image instability during xe2x80x9cdarkxe2x80x9d storage). Photosensitive silver halide remaining in the materials after imaging and development may continue to cause image formation. Such materials are exposed to a wide range of storage or transport temperatures after imaging. Under higher storage temperatures, post-processing instability and the resulting density changes may be even more pronounced over time, resulting in increases in Dmin (that is, fogging), or changes in density of the image.
Various compounds have been added to photothermographic materials as xe2x80x9cpost-processing stabilizersxe2x80x9d. Most often these are sulfur-containing compounds such as mercaptans, thiones, and thioethers. Specific classes of compounds that serve this purpose include the mercapto-substituted triazoles described in U.S. Pat. No. 5,149,620 (Simpson et al.), amido compounds described in U.S. Pat. No. 5,158,866 (Simpson et al.), azlactones described in U.S. Pat. No. 5,175,081 (Krepski et al.), alkyltriazoles described in U.S. Pat. No. 5,196,301 (Simpson et al.), compounds with nitrile blocking groups described in U.S. Pat. No. 5,300,420 (Kenney et al.), silyl-blocked compounds as described in U.S. Pat. No. 5,358,843 (Sakizadeh et al.), tribromosulfonyl compounds as described in U.S. Pat. No. 5,369,000 (Sakizadeh et al.) and U.S. Pat. No. 5,464,737 (Sakizadeh et al.), phthalimide-blocked compounds as described in U.S. Pat. No. 5,439,790 (Muthyala et al.), and mercapto-substituted tetrazoles as described in U.S. Pat. No. 5,681,693 (Bertoldi et al.).
While the noted compounds have provided a solution to the problem of post-processing instability, merely putting such compounds into photothermographic materials may not always provide optimum effectiveness. For example, some of the post-processing stabilizers have blocking groups that must be released during heat development to provide the desired effects. In some instances, there is inadequate release of the stabilizing moiety within the desired heat development time. Other known post-processing stabilizing compounds may contribute to fogging or cause a loss in photographic sensitivity, maximum density (Dmax), or contrast at the concentrations needed for post-processing stabilization.
Thus, there is a need in the industry to improve the post-processing stability of photothermographic materials under various storage conditions without any accompanying increase in fogging or unacceptable loss in photospeed and other sensitometric properties.
This invention provides a black-and-white photothermographic material that is sensitive at a wavelength greater than 700 nm, and comprises a support having thereon one or more thermally-developable imaging layers comprising a binder and in reactive association, a photosensitive silver halide, a non-photosensitive source of reducible silver ions, a reducing composition for the non-photosensitive source reducible silver ions, and a spectral sensitizing dye for the photosensitive silver halide that is a merocyanine dye or a cyanine dye that comprises one or more thioalkyl, thioaryl, or thioether groups,
wherein the one or more thermally-developable layers further comprises one or more indolenine dyes as post-processing stabilizing compounds.
Further, a method of this invention for forming a visible image comprises:
A) imagewise exposing the black and white photothermographic material described above to electromagnetic radiation at a wavelength greater than 700 nm to form a latent image, and
B) simultaneously or sequentially, heating the exposed photothermographic material to develop the latent image into a visible image.
In some embodiments of the imaging method of this invention, the photothermographic material has a transparent support and the imaging method further includes:
C) positioning the exposed and heat-developed photothermographic material between a source of imaging radiation and an imageable material that is sensitive to the imaging radiation, and
D) thereafter exposing the imageable material to the imaging radiation through the visible image in the exposed and heat-developed photothermographic material to provide an image in the imageable material.
The photothermographic materials of this invention exhibit improved post-processing stability after imaging. Thus, they are less likely to exhibit an increase in Dmin or change in density in the mid-tone regions when exposed to various temperatures during transport or storage of the imaged materials. The resulting images thus have improved aging characteristics. The photothermographic materials of this invention are also more useful in high temperature climates.
These advantages have been achieved by incorporating certain indolenine dyes into the thermally developable imaging layers on the frontside of the photothermographic materials as post-processing stabilizing compounds. The details of these dyes are provided below.